Several short peptidic compounds have been isolated from natural sources and found to have biological activity. Analogs of these compounds have also been prepared, and some were found to have biological activity. For example, Auristatin E (U.S. Pat. No. 5,635,483 to Pettit et al.) is a synthetic analogue of the marine natural product Dolastatin 10, an agent that inhibits tubulin polymerization by binding to the same site on tubulin as the anticancer drug vincristine (G. R. Pettit, Prog. Chem. Org. Nat. Prod., 70:1-79 (1997)). Dolastatin 10, auristatin PE, and auristatin E are linear peptides having four amino acids, three of which are unique to the dolastatin class of compounds. Both dolastatin 10 and auristatin PE are presently being used in human clinical trials to treat cancer. The structural differences between dolastatin 10 and auristatin E reside in the C-terminal residue, in which the thiazolephenethyl amine group of dolastatin 10 is replaced by a norephedrine unit in auristatin E.
The following references disclose dolastatin and auristatin compounds and analogs thereof, and their use for treating cancer:    International Publication No. WO 96/33212 A1 to Teikoku Hormone Mfg. Co., Ltd.;    International Publication No. WO 96/14856 A1 to Arizona Board of Regents;    European Patent Publication No. EP 695757 A2 to Arizona Board of Regents;    European Patent Publication No. EP 695758 A2 to Arizona Board of Regents;    European Patent Publication No. EP 695759 A2 to Arizona Board of Regents;    International Publication No. WO 95/09864 A1 to Teikoku Hormone Mfg. Co., Ltd.;    International Publication No. WO 93/03054 A1 to Teikoku Hormone Mfg. Co., Ltd.;    U.S. Pat. No. 6,323,315 B1 to Pettit et al.;    G. R. Pettit et al., Anti-Cancer Drug Des. 13(4): 243-277 (1998);    G. R. Pettit et al., Anti-Cancer Drug Des. 10(7): 529-544 (1995); and    K. Miyazaki et al., Chem. Pharm. Bull. 43(10), 1706-18 (1995).
Despite in vitro data for compounds of the dolastatin class and its analogs, significant general toxicities at doses required for achieving a therapeutic effect compromise their efficacy in clinical studies. Accordingly, there is a clear need in the art for dolastatin derivatives having significantly lower toxicity, yet useful therapeutic efficiency, compared to current dolastatin drug therapies.
The recitation of any reference in Section 2 of this application is not an admission that the reference is prior art to this application.